1. Field of the Invention
This invention relates generally to the acquisition of physiological data for health signs monitoring and, more particularly, for the diagnosis and treatment of sleep disorders.
2. Description of the Related Art
Sleep apnea (SA) is the most common disorder observed in the practice of sleep medicine and is responsible for more mortality and morbidity than any other sleep disorder. SA is characterized by recurrent failures to breathe adequately during sleep (termed apneas or hypopneas) as a result of obstructions in the upper airway.
Apnea is typically defined as a complete cessation of airflow. Hypopnea is typically defined as a reduction in airflow disproportionate to the amount of respiratory effort-expended and/or insufficient to meet the individual's metabolic needs. During an apnea or hypopnea, commonly referred to as a respiratory event, oxygen levels in the brain decrease, while the carbon dioxide (CO2) levels rise, causing the sleeper to awaken. The heart beats rapidly and blood pressure rises to high levels (up to 300 mm Hg). The brief arousals to breathe are followed by a return to sleep, but the apneas may recur over 60 times per hour in severe cases.
SA is a serious, yet treatable health problem for individuals worldwide. Published reports indicate that untreated SA patients are three to five times more likely to be involved in industrial and motor vehicle accidents and have impaired vigilance and memory. Studies show that more than 15% of men and 5% of women over the age of 30 and up to 30% of men and women over the age of 65 suffer from SA. SA during pregnancy is associated with hypertension and a risk of growth retardation in the fetus. Current estimates reveal that over 90% of individuals with moderate to severe SA remain undiagnosed.
A. Polysomnography
The current “gold standard” for the diagnosis of SA is an expensive (up to $2,000) overnight sleep study, called polysomnography (PSG), that is administered and analyzed by a trained technician and reviewed by a Board Certified Sleep Specialist. The limited availability of sleep centers coupled with the high capital expense to add capacity has resulted in a growing number of patients awaiting their PSG.
i. Data Recording
A conventional full overnight PSG includes recording of the following signals: electroencephalogram (EEG), submental electromyogram (EMG), electrooculogram (EOG), respiratory airflow (oronasal flow monitors), respiratory effort (plethysmography), oxygen saturation (oximetry), electrocardiography (ECG), snoring sounds, and body position. These signals are considered the “gold standard” for the diagnosis of sleep disorders in that they offer a relatively complete collection of parameters from which respiratory events may be identified and SA may be reliably diagnosed. The RR interval, commonly referred to as beats per minute, is derived from the ECG. Body position is normally classified as: right side, left side, supine, prone, or up (or sitting erect). Typically, the microphone and the body position sensor are taped over the pharynx. Each signal provides some information to assist in the visual observation and recognition of respiratory events.
Collapse of the upper airway is identified when the amplitude of the respiratory airflow and effort signals decrease by at least 50%, snoring sounds either crescendo or cease, and oxygen desaturation occurs. A respiratory event is confirmed (i.e., desaturation not a result of artifact) by the recognition of an arousal (i.e., the person awakens to breathe), typically identified by an increase in the frequency of the EEG, an increase in heart rate, or change in snoring pattern. The remaining signals assist in determining specific types of respiratory events. For example, the EEG and EOG signals are used to determine if a respiratory event occurred in non-rapid eye movement (NREM) or rapid eye movement (REM) sleep. The position sensor is used to determine if an airway collapse occurs only or mostly in just one position (typically supine).
ii. Identifying Respiratory Events
A reduction or absence of airflow at the airway opening defines sleep-disordered breathing. Absent airflow for 10 seconds in an adult is an apnea, and airflow reduced below a certain amount is a hypopnea. Ideally one would measure actual flow with a pneumotachygraph of some sort, but in clinical practice this is impractical, and devices that are comfortable and easy to use are substituted. The most widely used are thermistors placed in front of the nose and mouth that detect heating (due to expired gas) and cooling (due to inspired air) of a thermally sensitive resistor. They provide recordings of changes in airflow, but as typically employed are not quantitative instruments. Currently available thermistors are sensitive, but frequently overestimate flow. Also, if they touch the skin, they cease being flow sensors. Measurement of expired carbon dioxide partial pressure is used in some laboratories to detect expiration, but it is not a quantitative measure of flow.
An alternative method is to measure changes in pressure in the nasal airway that occur with breathing. This approach provides an excellent reflection of true nasal flow. A simple nasal cannula attached to a pressure transducer can be used to generate a signal that resembles that obtained with a pneumotachygraph. It allows detection of the characteristic plateau of pressure due to inspiratory flow limitation that occurs in subtle obstructive hypopneas.
An obstructive apnea or hypopnea is defined as an absence or reduction in airflow, in spite of continued effort to breathe, due to obstruction in the upper airway. Typical polysomnography includes some recording of respiratory effort. The most accurate measure of effort is a change in pleural pressure as reflected by an esophageal pressure monitor. Progressively more negative pleural pressure swings leading to an arousal have been used to define a “Respiratory Effort Related Arousal” (RERA), the event associated with the so-called “Upper Airway Resistance Syndrome”. However the technology of measuring esophageal pressure is uncomfortable and expensive, and rarely used clinically. Most estimates of respiratory effort during polysomnography depend on measures of rib cage and/or abdominal motion. The methods include inductance or impedance plethysmography, or simple strain gages. Properly used and calibrated, any of these devices can provide quantitative estimates of lung volume change and abdominal-rib cage paradox. However calibrating these devices and keeping them accurately calibrated during an overnight recording is very difficult and as a practical matter is almost never done. Thus the signals provided by respiratory system motion monitors are typically just qualitative estimates of respiratory effort.
B. Measuring Oxyhemoglobin Desaturation During Sleep
One of the functions of the lungs is to maintain a normal partial pressure (tension) of oxygen and carbon dioxide in the arterial blood. Various dynamic processes, such as ventilation, diffusion, and the matching of ventilation and perfusion within the lung support this function. Ventilation or breathing, for example, continuously replenishes the oxygen (O2) in the gas-exchanging units of the lung, the alveoli, and removes carbon dioxide (CO2). An apnea or hypopnea occurring during sleep, however, temporarily decreases alveolar ventilation, causing a drop in arterial oxygen tension (pO2) and an increase in arterial carbon dioxide tension (pCO2). Because there is currently no accurate non-invasive method for continuously monitoring arterial pO2 or pCO2, non-invasive measures of oxyhemoglobin percent saturation are instead used today to determine apneas or hypopneas.
Blood transports oxygen both as dissolved O2 and in chemical combination with hemoglobin. The amount of dissolved O2 is directly proportional to the partial pressure of O2. At atmospheric pressure, the amount of dissolved O2 accounts for only a trivial amount of the blood oxygen content, not nearly enough to sustain life. Each gram of hemoglobin can carry up to 1.34 ml of O2. Oxyhemoglobin percent saturation (saturation) is the ratio of the amount of O2 actually combined with hemoglobin in the red blood cells to the maximum capacity of that hemoglobin to bind O2, expressed as a percent. At sea level, a healthy person typically has a pO2 of about 100 mmHg and saturation between 97% and 98%, or on average 97.4%.
The amount of O2 combined with hemoglobin is not linearly related to O2 tension, pO2. A graph of saturation against pO2 is a sigmoid curve that has a steep slope between a pO2 of 10 and 50 mmHg, and a very flat portion between 70 and 100 mmHg. The relationship between saturation and pO2 at the top of the curve is optimal for getting oxygen from the lungs to the tissue, but makes detecting small drops in arterial pO2 difficult. When oximetry is used to identify decreases in ventilation occurring as the result of upper airway collapse in persons with sleep apnea, the non-linear characteristics of the curve are particularly relevant. This is because desaturations resulting from sleep apnea occur most frequently in the range between 88% and 98%, the flat portion of the curve. The American Academy of Sleep Medicine Task Force recently established one of the main criteria for identifying a sleep apnea/hypopnea: desaturation>3% lasting a minimum of 10 seconds. See the Report of the American Academy of Sleep Medicine Task Force: Sleep-Related Breathing Disorders in Adults: Recommendations for Syndrome Definition and Measurement Techniques in Clinical Research, Sleep, Vol. 22, No. 5, 1999. It might be said that this report reflects a pessimistic view of the accuracy of the usual pulse oximetry, not physiology. Defining a respiratory event by a fixed change in saturation without defining the starting saturation, however, does not make biological sense. For example, a 3% fall in saturation from 98% is a drop in pO2 of 38 mmHg, while a 3% decrease from 94% reflects a 9-mmHg pO2 change.
C. Oximetry
The measurement of oxyhemoglobin saturation using pulse oximetry was developed in the 1940s, but became practical and universally available with the availability of microprocessors. A pulse oximeter typically utilizes two different light sources (e.g., red and infrared), which measure different absorption or reflection characteristics for oxyhemoglobin (i.e., the red, saturated blood) and deoxyhemoglobin (the blue, unsaturated blood). The oximeter then measures the ratio (percent) of saturated to unsaturated hemoglobin. One method to determine blood oxygen saturation is by transmission oximetry. Devices utilizing transmission oximetry operate by transmitting light through an appendage, such as a finger or an earlobe, and comparing the characteristics of the light transmitted into one side of the appendage with that detected on the opposite side. Another method to determine blood oxygen saturation is by reflectance oximetry, which uses reflected light to measure blood oxygen saturation. Reflectance oximetry is useful to measure oxygen saturation in areas of the patient's body that are not well suited for transmission measurement. See, for example, the description in U.S. Pat. No. 4,796,636 to Branstetter and Edgar.
Pulse-oximeter devices commonly used for the diagnosis of sleep apnea were originally designed to monitor patients in critical care conditions, even though the requirement for optimal sensitivity for the two applications differs. In critical care monitoring, the device is typically calibrated to set off an alarm and notify hospital staff when a patient's saturation falls below a certain critical threshold (e.g., 88%). Averaging the data across a wider time window (e.g., five seconds) is a common technique embedded in the device to minimize false alarms due to measurement artifacts. Studies have shown that the sensitivity of a pulse-oximeter to subtle fluctuations in oxygen saturation due to partial obstruction of the pharynx is directly related to the averaging window that is employed. See, for example, “Oximeter's Acquisition Settings Influence the Profile of the Respiratory Disturbance Index” by Davila D, et al. in Sleep 2000: 23; Abstract Supplement 2 at A8-A9. A calibration curve developed to optimize the accuracy of the oxygen saturation measurements across the typical specified range (i.e., from 100 to 70%), a requirement in critical care situations, may reduce the accuracy of the measures at more subtle resolutions (e.g., 98.0 to 97.5%). In monitoring a sleeping person, however, a repetitive pattern of oxygen desaturation between 98% and 96% that is terminated by an arousal is significant, though commonly overlooked due to the insensitivity of existing devices.
Most commercial pulse-oximetry sensors in use today are designed to be taped or affixed to the body with a wire lead that is inserted into the pulse-oximeter monitoring equipment. This wire lead, however, is one of the main sources of measurement artifacts. During sleep, the wire can get caught in the patient's bedding, thus causing a disruption of the sensor contact with the skin, where the red and infrared light sources are being measured. Furthermore, in many conventional in-home systems used to determine or treat apnea, the patient is required to apply sensors, plug in wires, apply and adjust transducers, straps, gauges, or other measurement devices, or operate a computer-controlled bedside unit. This equipment can be difficult for a lay person to apply and properly operate. Thus, a device that eliminates or reduces the use of wires, and can be reliably self-applied with minimal instruction would be beneficial in the accurate diagnosis of sleep disorders.
D. Continuous Positive Airway Pressure (CPAP)
Sleep apnea treatment is widely available and relatively inexpensive. Patients can be fitted with a Continuous Positive Airway Pressure (CPAP) device, which delivers air at a constant increased pressure via a nasal mask worn throughout the night. This increased pressure propagates through the nose into the pharynx and prevents the airway from collapsing.
CPAP is the most common treatment for obstructive sleep apnea (OSA). OSA is characterized by frequent periods of airway occlusion during sleep, with concomitant obstruction of inspiratory airflow, drop in blood oxygen, and interruption of sleep when the patient awakes to use voluntary muscle contraction to open the airway and take a few deep breaths. Typically a patient diagnosed with clinically important sleep apnea/hypopnea will undergo a CPAP titration and trial during attended PSG. A technician assists the patient with the fitting of the CPAP mask and determines the pressure required to keep that patient's airway open during sleep. Recent developments in CPAP technology include auto-titration units, which automatically adjust the pressure to that required at any particular time during sleep. These auto-titration CPAP units, however, are more expensive than standard CPAP devices and not usually reimbursed by medical insurance. Their application is generally limited to unattended automatic determination of required pressure, rather than relying on a technician to determine the pressure needed during PSG.
Some states, including Alabama, require employers to monitor the compliance of CPAP use for truck drivers diagnosed with sleep apnea. A recent innovation of the CPAP technology includes a “smart system” to monitor compliance by recording and storing the time the CPAP device is on at the prescribed pressure. A way to inexpensively monitor treatment outcomes, improve the titration of CPAP devices, and improve compliance is thus desirable.
E. Neuromuscular Stimulation
A number of recent developments in the area of treating sleep apnea suggest that neuromuscular stimulation may be appropriate for the treatment of sleep apnea. See, for example, U.S. Pat. No. 6,240,316 to Richmond and Loeb (hereinafter referred to as Patent '316), U.S. Pat. No. 5,549,655 issued to Erickson (hereinafter referred to as Patent '655), and U.S. Pat. No. 5,291,216 (hereinafter referred to as Patent '216). One of the preferred embodiments described by Patents '316 and '655 includes an open loop system, whereby stimulation is timed to the patient's respiration. Patent '316 describes a method for sensing obstructed airway passage by sensing airway pressure, characteristic snoring sounds, mechanical motion, or muscle activity. Patent '655 describes stimulation of the upper airway using a measurement of respiratory effort. Patent '216, on the other-hand, describes that the placement of the neuro-stimulation electrode may be sufficient to maintain upper airway passage. More effective treatment of sleep apnea could be provided if improved detection of sleep apnea events were coupled with the delivery of stimuli provided by such neuromuscular devices.
From the discussion above, it should be apparent that there is a need for a more efficient, inexpensive, and accurate way to collect physiological data to detect sleep related obstructive respiratory events, as well as address the difficulties and problems discussed above. The present invention fulfills these needs.